1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier or a printer that obtains an image by using a toner to visualize an electrostatic image formed on an image bearing member. More specifically, the present invention relates to an image forming apparatus that employs as its developer a dual-component developer which has a toner and a carrier.
2. Description of the Related Art
In conventional copiers, printers, and other image forming apparatuses that use an electrophotographic process, a surface of an electrophotographic photosensitive member (hereinafter simply referred to as “photosensitive member”) serving as an image bearing member is charged uniformly, and the surface is then exposed to light in a pattern determined by image information. An electrostatic image (latent image) is thus formed on the surface of the photosensitive member. The electrostatic image formed on the photosensitive member is developed as a toner image by a developing device with the use of a developer. The toner image formed on the photosensitive member is transferred to a transfer material directly or through an intermediate transfer member. The toner image is then fixed to the transfer material, to thereby obtain a recorded image.
There are roughly two types of developers: mono-component developers which substantially consist of toner particles alone and dual-component developers which contain toner particles and carrier particles. Generally speaking, a developing method that uses a dual-component developer has advantages over one that uses a mono-component developer in that it is capable of forming a higher definition image in truer colors.
In an ordinary dual-component developer, magnetic particles (carrier) about 5 μm to 100 μm in diameter and particles of a non-magnetic toner about 1 μm to 10 μm in diameter are mixed at a given mixture ratio. The function of the carrier is to carry the charged toner to deliver the toner to a developing portion. The toner is charged with a given amount of electric charges of a given polarity through frictional charging by being mixed with the carrier.
Along with progress in terms of digitization, a pursuit of full-color, and speeding up of copiers, printers, and other image forming apparatuses that use a photographic process, their output images have recently come to be valued as original output materials, and there is even a great expectation on their entry into the printing market. Photographic process image forming apparatuses are therefore required to be capable of outputting images of higher quality (higher definition) steadily without allowing the image quality to fluctuate. To attain an image quality of that high definition, improving the development property is essential.
In a development process that uses a dual-component developer, the dual-component developer is usually carried on a developer carrying member in a developing device and transported to a developing portion, which faces an electrostatic image on a photosensitive member. The magnetic brush of the dual-component developer on the developer carrying member are brought into contact with, or close to the photosensitive member. The toner alone is then transferred to the photosensitive member by a given level of developing bias applied between the developer carrying member and the photosensitive member. A toner image corresponding to the electrostatic image is thus formed on the photosensitive member.
The developing bias that is widely employed is an alternating bias in which a DC voltage component and an AC voltage component are superimposed. The development property is improved when more toner particles are pulled apart from the carrier and put to use in the developing method. To accomplish this, the toner needs to be subjected to a higher electric field intensity.
A quick way to enhance the intensity of the electric field applied to the toner is to simply apply a higher level of developing bias between the developer carrying member and the photosensitive member. However, increasing the developing bias to a level higher than necessary may cause an injection of electric charges from the developer carrying member into the electrostatic image through the carrier, which disturbs the electrostatic image.
A conventionally popular photosensitive member is an organic photoconductor (OPC) photosensitive member in which a charge generation layer made up of an organic material, a charge transport layer, and a surface protecting layer are layered on a metal base.
On the other hand, it is a known fact that a single-layer photosensitive member, such as an amorphous silicon photosensitive member (hereinafter referred to as “a-Si photosensitive member”), is effective for forming an electrostatic image that has as high a resolution as described above. One of the reasons is as follows.
The interior charge generating mechanism of an a-Si photosensitive member is on the surface of the photosensitive member, whereas the interior charge generating mechanism of an OPC photosensitive member is located near the base of the photosensitive member. This prevents electric charges generated inside an a-Si photosensitive member from diffusing before reaching the surface of the photosensitive member, and an electrostatic image of extremely high definition is obtained as a result.
A drawback of a-Si photosensitive members is that their surface resistance is lower than that of OPC photosensitive members, which makes the influence of the above-mentioned charge injection from the developer carrying member through the carrier in a-Si photosensitive members much greater than the one in OPC photosensitive members. Therefore, when an a-Si photosensitive member is employed, a formed electrostatic image can easily be disturbed by the charge injection and the traveling of electric charges has to be restricted even more than when an OPC photosensitive member is employed by lowering the peak-to-peak voltage, Vpp, of the developing bias, which is alternating bias.
Lowering Vpp of the developing bias reduces electric charges injected from the developer carrying member to the photosensitive member through the carrier, but weakens the electric field applied to the developer. Accordingly, the force to detach the toner from the carrier is reduced and the development property is lowered.
Setting the electric resistance of the carrier is effective for forming a high quality image as proposed in Japanese Patent Application Laid-Open No H08-160671.
However, setting the electric resistance of the carrier high is known to tend to lower the development property, in other words, the ability to detach (discharge) the toner from the carrier.
As described above, the carrier in a dual-component developer has a role of charging the toner by frictional charging in addition to the role of carrying the toner to the developing portion. The carrier is therefore charged with electric charges having a polarity reverse to that of the electric charges, with which the toner is charged. For instance, when the toner is charged with negative electric charges, the carrier is charged with positive electric charges.
In charging the toner, the electric resistance of the carrier set high makes it difficult for electric charges accumulated in the carrier to travel. The electric charges in the carrier and electric charges in the toner thus attract each other, thereby generating a large attractive force and hindering the toner from detaching from the carrier. The electric resistance of the carrier set low makes it easy for electric charges inside the carrier to diffuse on the surface of the carrier, thereby reducing the attractive force between the toner and the carrier and facilitating the detachment of the toner from the carrier.
Other methods of enhancing the electric field intensity to which the toner is subjected than increasing the developing bias applied between the developer carrying member and the photosensitive member include raising the permittivity of the carrier. When the permittivity of the carrier is high, polarized charges generated inside the carrier reduce the potential difference within the carrier and the electric field concentrates correspondingly on an air space between the carrier on the photosensitive member side and the photosensitive member. The toner adhering to the carrier will accordingly be subjected to an enhanced electric field intensity.
Raising the permittivity of the carrier is considered to facilitate the removal of even the toner once carried to the photosensitive member so that the development property is lowered.
As mentioned above, alternating bias in which a DC voltage component and an AC voltage component are superimposed is employed as the developing bias applied between the developer carrying member and the photosensitive member. When the developing bias is applied in a direction that moves the toner to the photosensitive member (hereinafter referred to as “development direction bias”), the toner is pulled apart from the carrier and transported to the photosensitive member. When the alternating bias is switched to apply the developing bias in a direction that moves the toner to the developer carrying member (hereinafter referred to as “pull-back direction bias”), the toner is transported toward the developer carrying member.
First, when the development direction bias is applied, the electric field intensity to which the toner is subjected is higher and more toner particles are detached from the carrier to be transported to the photosensitive member with a high permittivity carrier A than with a low permittivity carrier B from the reason described above. Also when the alternating bias is switched to apply the pull-back direction bias, the toner is subjected to a higher electric field intensity and more toner particles are detached from the photosensitive member with the high permittivity carrier A than with the low permittivity carrier B, which is inconvenient in that the influence of the permittivity on the development property is weakened.
FIG. 15 illustrates a development property difference between cases in which two types of conventional ordinary carrier having different permittivity characteristics (high permittivity carrier A and low permittivity carrier B) are employed. In FIG. 15, the axis of abscissa illustrates the peak-to-peak voltage Vpp of the developing bias and the axis of ordinate illustrates a per-unit area charge amount Q/S [C/cm2] of a toner layer of a toner image formed on the photosensitive member. Q/S [C/cm2] is a value calculated by multiplying a per-unit toner weight charge amount Q/M [μC/g] of the toner layer on the photosensitive member at which the maximum density is obtained by a per-unit area toner bearing amount M/S [mg/cm2] of the toner layer. The Q/S [C/cm2] indicates the developing performance of the developer, in other words, how much of the toner has been migrated onto the photosensitive member by overcoming the attractive force between the carrier and the toner. The maximum density is the density of a solid image and, in the case of reversal development, an image density at which the potential difference between the DC component of the developing bias and the electric potential of an image portion of the photosensitive member is maximum.
Illustrated in FIG. 15 are results that are obtained when the photosensitive member employed is an OPC photosensitive member 30 μm in film thickness (thickness of the photosensitive layer).
It is understood from FIG. 15 that Q/S [C/cm2] is higher with the high permittivity carrier A than with the low permittivity carrier B regardless of the Vpp level of the developing bias. FIG. 4 illustrates the electric field dependencies of the permittivities of the high permittivity carrier A and the low permittivity carrier B. The permittivity of a carrier has characteristics that vary depending on the electric field applied to the carrier. In FIG. 4, the permittivity of the high permittivity carrier A is higher than that of the low permittivity carrier B in both the development direction bias and the pull-back direction bias. Yet, Q/S [C/cm2] is higher with the high permittivity carrier A than with the low permittivity carrier B as illustrated in FIG. 15 because the influence of the permittivity upon application of the development direction bias over the electric field intensity for moving the toner to the photosensitive member is larger than the influence of the permittivity upon application of the pull-back direction bias over the electric field intensity for pulling the toner apart from the photosensitive member. Therefore, because of the electric field intensity difference caused by the difference in permittivity, the development property is better with the high permittivity carrier A than with the low permittivity carrier B.
The development property is also greatly influenced by the capacitance of the photosensitive member. The development property degrades as the capacitance (per-unit area capacitance) of the photosensitive member increases and, when the degradation progresses beyond allowable limits, various image defects occur. The relation between the capacitance of the photosensitive member and the development property is described next.
Take as an example a case where a maximum density toner image is formed on the OPC photosensitive member under the following conditions; Development contrast (potential difference between the electric potential of the image portion on the photosensitive member and the DC voltage of the development bias)
Vcont=250 V
Toner charge amount Q/M=−30 μC/g
Toner bearing amount M/S=0.65 mg/cm2 
An electric potential (charging potential) ΔV produced by a toner layer of this toner image on an OPC photosensitive member having a film thickness of 30 μm is calculated by the following equation:
                                          Δ            ⁢                                                  ⁢            V                    =                                                                                          ɛ                    t                                    ⁢                                      ɛ                    0                                                                    2                  ⁢                                                                          ⁢                  λ                  ⁢                                                                          ⁢                  t                                            ⁢                              (                                  Q                  S                                )                                      +                                                                                ɛ                    d                                    ⁢                                      ɛ                    0                                                                    d                  th                                            ⁢                              (                                  Q                  S                                )                                                    ⁢                                  ⁢                              where            ⁢                                                  (                          Q              S                        )                    =                                    (                              Q                M                            )                        ×                          (                              M                S                            )                                                          Equation        ⁢                                  ⁢        1            
Q/M represents the per-unit weight toner charge amount on the photosensitive member.
M/S represents the per-unit area toner weight of a maximum density portion on the photosensitive member.
λt represents the toner layer thickness of the maximum density portion on the photosensitive member.
dth represents the film thickness of the photosensitive member.
∈t represents the relative permittivity of the toner layer.
∈d represents the relative permittivity of the photosensitive member.
∈0 represents the permittivity of a vacuum.
Under the above conditions, ΔV=243 V and fills Vcont=250 V. In other words, electric charges in the toner layer satisfactorily fill the electric potential of the electrostatic image (charging efficiency: 97%).
The material characteristics of a-Si photosensitive members are such that their relative permittivity is about three times larger than that of OPC photosensitive members (a-Si photosensitive members: approximately 10, OPC photosensitive members: approximately 3.3). Accordingly, when an a-Si photosensitive member and an OPC photosensitive member have the same film thickness (30 μm, for example), the capacitance of the a-Si photosensitive member (e.g., 2.95×10−6 F/m2) is about three times larger than that of the OPC photosensitive member (e.g., 0.97×10−6 F/m2).
Consider a case of forming a maximum density toner image on an a-Si photosensitive member under the same conditions as in the above example where an OPC photosensitive member is employed, where the Vcont is 250 V and the toner charge amount Q/M is −30 μC/g. From the above equation, a toner amount necessary in this case to satisfy ΔV=250 V is 1.15 mg/cm2, which means that the amount of the toner to be migrated onto the a-Si photosensitive member is about 1.7 times the amount of the toner on the above OPC photosensitive member. Conversely, the a-Si photosensitive member needs an about 1/1.7 of the development contrast of the OPC photosensitive member to obtain a toner bearing amount M/S of 0.65 mg/cm2. An a-Si photosensitive member accordingly needs a development contrast Vcont of about 147 V to fill electric charges of a high density portion.
However, in the quick printing market or the like where a wide range of tone reproduction is required, the γ characteristic (characteristic of the image density in relation to the image exposure amount) at Vcont=147 V may be too sharp to attain a high tone reproduction property, with the result that a halftone image such as a photographic image is difficult to be reproduced.
Attempts to reduce the film thickness (photosensitive layer thickness) of OPC photosensitive members have been made in order to sharpen the electrostatic image. Also in those cases, a reduction in film thickness of the photosensitive member causes an increase in capacitance of the photosensitive member, which may cause the same problem as the one described above regarding a-Si photosensitive members.
A possible way to deal with the problem that arises from setting the relative permittivity of the photosensitive member high or reducing the film thickness of the photosensitive member is to increase Q/S [C/cm2] of the toner layer of the toner image, in other words, to increase the toner charge amount Q/M [μC/g]. For instance, the toner charge amount Q/M [μC/g] is changed from −30 μC/g of the above example to −60 μC/g. In this state, if a toner bearing amount M/S [mg/cm2] of 0.65 mg/cm2 is obtained at a development contrast Vcont of, for example, 240 V, the electric potential ΔV produced by the toner layer is 238 V (that is, approximately 240 V) and the charging efficiency is approximately 100%.
In practice, however, increasing the toner charge amount Q/M [μC/g] increases the electrostatic force of the carrier and the toner significantly, and may seriously degrade the development property.
As has been described, with a-Si photosensitive members and other photosensitive members that have a low surface resistance, Vpp of the developing bias cannot be increased because the injection of electric charges into the electrostatic image during development has to be avoided. With a-Si photosensitive members, thin film OPC photosensitive members, and other photosensitive members that have a large capacitance, setting the toner charge amount Q/M [μC/g] high is effective in obtaining a stable and satisfactory tone reproduction property while avoiding such image defects as blank spots, except that, in some cases, setting the toner charge amount Q/M [μC/g] high seriously degrades the development property.